Conventionally, a ceramic metal circuit board having an insulation and electrode function has been widely used in a field of mounting power electronics. In such field, a substrate which is mainly composed of alumina (Al2O3) or aluminum nitride (AlN) as a main component has been conventionally used as a ceramic substrate.
However, since a thermal conductivity of an alumina substrate is as low as about 18 W/m·K, its heat dissipation property (heat radiating property) is insufficient. On the other hand, although the thermal conductivity of an AlN substrate is as high as about 200 W/m·K, the mechanical strength of the AlN substrate is low, and hence its heat-cycle resistance characteristics are insufficient.
To cope with these problems, a high thermal conductive silicon nitride substrate has been developed as a ceramic material having both an excellent thermal conductivity characteristic and an excellent mechanical strength characteristic. For example, Japanese Patent Laid-Open No. 2009-120483 (Patent Document 1) discloses a metal circuit board which is made from silicon nitride ceramic substrate and in which leakage current is reduced by controlling a diameter of pores formed in a grain boundary phase of the silicon nitride ceramic substrate.
Meanwhile, the metal circuit board made from silicon nitride ceramic sintered body is formed by bonding a copper circuit plate to a silicon nitride substrate via (through) an Ag—Cu—Ti-based active metal brazing material. The silicon nitride substrate contains silicon nitride as its main component, and hence has a three-point bending strength as high as 600 MPa or more. For this reason, the bonding structure of the silicon nitride substrate and the copper plate also has excellent heat-cycle resistance characteristics, and hence, even when the bonding structure is subjected to heat cycles for a long period of time, defects, such as a crack and peeling, are hardly caused in the silicon nitride substrate.
For example, Japanese Patent Laid-Open No. 2003-192462 (Patent Document 2) discloses that the metal circuit board made from silicon nitride substrate and obtained according to Patent Document 2 can withstand a heat-cycle resistance test (TCT test) of 3000 cycles.
On the other hand, a bonding process is necessary to form a bonded body of a ceramic substrate and a metal circuit plate, which inevitably results in an increase in manufacturing costs. For this reason, as disclosed in Japanese Patent Laid-Open No. 2003-197836 (Patent Document 3), for the purpose of improving the insulating property, it has been proposed to use a silicon nitride substrate as a spacer for a pressure contact structure. Further, since the silicon nitride substrate has high mechanical strength and a high fracture toughness value, it has been confirmed that the silicon nitride substrate can also sufficiently withstand stress generated in the case where the substrate is applied to a pressure contact structure using screws and the like.
A silicon nitride sintered body, which constitutes the silicon nitride substrate, contains a β-silicon nitride (Si3N4) as a main phase. A β-Si3N4 particle (grain) is a crystal particle having an elongated shape in which the ratio of the major axis length to the minor axis length (aspect ratio) is two or more. In the silicon nitride sintered body, a structure having high mechanical strength and a high fracture toughness value is realized in such a manner that a large number of β-Si3N4 particles having an average particle diameter of about 2 to 10 μm are complicatedly entangled with each other.
As described above, the silicon nitride substrate contains β-Si3N4 particles as a main phase, and hence microscopic depressions and projections exist on a surface of the silicon nitride substrate. This is caused by the fact that β-Si3N4 particles are complicatedly entangled with each other. Even when the surface of the silicon nitride substrate is mirror-polished so as to have a surface roughness Ra of 0.05 μm or less, it is difficult to eliminate these depressions and projections. The mirror polishing process itself also causes an increase in the manufacturing costs.
In a silicon nitride substrate having microscopic depressions and projections described above, especially in a silicon nitride substrate having a projecting portion, there may arise a problem in that, when the silicon nitride substrate is used for a long period of time under application of pressure contact stress, a crack is caused in the silicon nitride substrate with the projecting portion as a starting point.
Further, as described above, the microscopic depressions and projections exist on the surface of the silicon nitride substrate, and hence in the pressure contact structure, microscopic gaps are caused between the silicon nitride substrate and a member (contact member) which is brought into contact with the silicon nitride substrate. The contact member is generally constituted of a metal member, such as a metal plate. Therefore, when gaps are formed between the silicon nitride substrate and the metal member due to the microscopic depressions and projections on the surface of the silicon nitride substrate at the time of forming the pressure contact structure, the gaps impede heat conduction between the silicon nitride substrate and the metal member, which results in a deterioration of heat dissipation characteristics (heat radiation properties) of the pressure contact structure as a module.